12 
INSECTS IN GENERAL 
forms a kind of wall behind the mouth, usually deeply notched iu the 
middle, and which is called the mentum, or chin. 
When the labium forms a narrow elongated piece, distinct from the 
mentum, as in most of the Goleoptera, it is now generally called the 
tongue, lingua or ligula. 
The Palpi , or appendages of the mouth. — Near the base of each max- 
illa, on its outer side, is attached a movable appendage, usually com- 
posed of four or five joints, and never more than six, called the maxil- 
lary palpus ; and near the base of the labium is attached a similar pair 
of organs, but with a less number of joints, distinguished as the labial 
palpi. These appendages are subject to considerable variation espe- 
cially iu the shape of their terminal joints, and are made much use of 
in determining the families and genera of insects. 
The liaustellate or suctorial mouth consists of a more or less elongated 
proboscis or sucker, which is sometimes short and fleshy, as in the flies, 
(Muscidw;) sometimes more elongate, horny and pointed, as in the 
bugs, (Hcmiptera ;) and sometimes very long and slender, and rolled 
up, when not iu use, in a spiral coil, as in the butterflies and moths, 
(Lepidoptera.) 
It is evident that all insects with a suctorial mouth must live exclu- 
sively upon liquid food, or the juices of animals and plants. 
The haustellum or sucker is not a single organ, as it appears, but has 
upon its upper side a deep groove, in which are contained usually either 
two or four, but in some of the carnivorous species (mosquitoes and 
horse-flies) six needle-shaped pieces, which iu these last make a compli- 
cated weapon with which they pierce the skins of animals upon whose 
blood they subsist. 
From a comparison of the liaustellate with the mandibulate mouth, in 
different kinds of insects, it has been concluded that the apparent sucker, 
which, as we have just seen, forms a sheath for the smaller needle- 
shaped pieces, corresponds to the labium, and that the contained pieces 
must represent the mandibles and maxillae, and, where six pieces are 
present, also the labrum and lingua. In accordance with the propor- 
tionately great development of the labium, we find that its appendages, 
that is, the labial palpi, are also very prominent, whilst the maxillary 
palpi are very small or rudimental. This is the case in two of the suc- 
torial orders, the Lepidoptera and Diptera; but the other order (ilemip- 
tera) is exceptional in this respect, having neither maxillary nor labial 
palpi developed. 
THE THORAX AND ITS APPENDAGES. 
The thorax is the second, or middle division of the bodies of insects. 
Though apparently single, it is really composed of three pieces soldered 
